Miata suspension bumpstops

Here I have collected some measurements and information about the
Miata bumpstops and their role in the suspension setup. This page was
a subsection of my Ground Control setup page but
it grew enough to be useful in its own right regardless of which
spring kit is being used.

Effect of bump stops

Before I started experimenting with the GC installation I assumed
the bumpstop were a more or less solid chunk of rubber whose role was
merely to protect the shock from hitting metal when the suspension
bottomed out severly due to potholes or similar anomalies. I assumed
that the shock would never ride or touch these under normal
cirumstances, whether commuting or performance driving.

Turns out that was all wrong. The bump stops are an integral part
of the suspension travel in the Miata. They offer a progressive spring
rate, additive to whatever spring rate the spring itself may
have. Finding a ride height which does not "bottom out" onto the
bumpstops (which I tried to do for the initial coilover installation)
is not only not necessary, but impossible.

A Miata even with its stock suspension corners on the bumpstops
essentially all the time. While driving in a straight line, the soft
springs provide a nice ride. As soon as cornering forces go beyond
suburban commuting, the bump stops add from several hundred to a
thousand lbs/in of spring rate to the equation, giving nice handling.

So what is the spring rate of the bump stops? In the Jan/Feb 1998
issue of GRM (p.17), it states that the Miata bump stops have the
following spring rates. However, take these figures with a grain of
salt. I am not sure what they were measuring, but the entire bumpstop
is less than four inches long, so these numbers must be wrong in some
way?

During first inch of compression: 150 lbs/in
During second inch of compression: 250 lbs/in
During third inch of compression: 550 lbs/in
During 4th inch of compression: 1000 lbs/in

In a
posting to the miatapower list it states that
from experimentation the spring rates appear to start out much softer,
around 40in/lb, going up to 1000 or so (as stated above).

New in 2006: Shaikh
gave me a graph with partial measurements from a 'new' OEM bumpstop
(see brown triangles).

The different numbers may be explained in part by the fact that
there are two types of bumpstops used in Miatas. What I'll call the
'old' and 'new'. The 'old' (NA01-28-111) ones were used in 90-93 1.6l
Miatas. The 94+ 1.8l cars use the 'new' (N021-34-111) ones in the
front and the 'old' ones in the rear. The R package cars, as well as
the '93LE, use the 'new' bumps in all four corners. (This is on
USA-spec Miatas, others may be different?). Thanks to Tommy [aka Parts
Dood] for this info.

Below are the two bumpstops with some measurement data. All
dimensions are in mm.

Fig#1: Drawing of 'old' bumpstop

Fig#2: Drawing of 'new' bumpstop

As you can see, the 'new' bumpstop is a good bit longer. Presumably
it offers a larger range of spring rate, starting out softer. Keep
these differences in mind when comparing setups if the cars don't have
the same style bumpstop.

How much travel?

A question that seems to come up often is how much travel there
really is available before the shock contacts the bumpstop. The
following table shows the distance from the shock to the bumpstop at
various ride heights. The shock is a Bilstein, and the measurements
were taken with an 'old' style bumpstop. Substract 13mm from the
available travel if you are using the 'new' style bumpstops. Ride
height is measured from fender lip to hub center. The picture above is
the rear suspension at 14" ride height.

Ride height

Rear

Front

14"

1.0625" (27mm)

1.125 (28.6mm)

13.5"

not measured

0.5625 (14.3mm)

13"

0.4375" (11mm)

0.375 (9.5mm)

12.75"

0 (light contact)

not measured

12.5"

squeezed

0 (light contact)

12"

squeezed

squeezed

Rear 14" ride height

Rear 12.75" ride height

Rear 12" ride height

Front 13.5" ride height

Front 12.5" ride height

How much to cut?

It is easy to conclude from the above pictures and numbers that if
the car is to be lowered by any non-trivial amount, the bumpstops
probably need to be cut. With 'old' style bumpstops a ride height any
lower than 12.5F/12.75R will be permanently riding on the bumps, quickly
leading to a very harsh ride on anything but glass-smooth roads.

But what happens if the bumpstops are cut too much as was the case
with my initial GC install? My rear bumpstops were cut nearly to
nonexistence. I only had the top hat portion of the bumpstop. Looking
at Figure #1 above, only the top 26mm section was left. (In the front
I had 41mm, consisting of the 'hat' plus the first 'donut'). The
result of this is that I had more travel before the shock contacted
the bump. However, once it did bottom out, the spring rate went from
450lbs/in (of the spring) to somewhere over 1000lbs/in in a very short
distance of suspension travel. This non-linear increase could be felt
at the track, as the rear end of the car would suddenly stiffen up
considerably.

For predictable handling the goal should be to achieve a reasonably
progressive spring rate vs. travel graph. My suggestion is to draw
such a travel vs. rate graph before deciding on spring rates and ride
heights and how much to cut off the bumpstops. Unfortunaly there is no
precise data on the bumpstop spring rates, but the numbers quoted
above should provide a starting place at the very least.

Based on the above analysis I decided to keep full length 'new'
style bumpstops and a stock ride height. As documented in my GC page, the results have been very good.